1. Field of the Invention
The present invention relates to an organic light emitting device (OLED) in which a charge transport material is used as the host material and/or as the charge transport layer.
2. Description of the Related Art
Significant efforts have been expended in developing suitable materials for use in organic light emitting devices (OLEDs). Such devices are commercially attractive because they offer the promise of low-cost fabrication of high-density pixeled displays exhibiting bright electroluminescence with long life times, high efficiency and wide color range.
A typical OLED is fabricated by sandwiching an emissive layer between an anode and a cathode. Improved performance can be obtained by the provision of additional layers around the emissive layers so as to provide charge transport capabilities, such as an electron transport layer or a hole transport layer.
The stability and lifetime of an OLED may change with the various combinations of emissive material and charge transport material. As most luminescent materials have limited charge injection and transporting ability, and/or unbalanced charge injection ability in the device, the doping of emissive material (with a volume content less than 20%) into a host material can lead to much better performance because the host material can either enhance electron transport or enhance hole transport. Since electron injection of organic luminescent material has been less efficient than hole injection of organic luminescent material, host materials that have an electron enhancement function, have been widely used for high performance OLEDs. For example, U.S. Pat. No. 5,935,720 shows an OLED utilizing tris(8-hyroxyquinoline) aluminum Alq3 as host material because of its electron enhancement function. Due to its high electron affinity, Alq3 has also been widely explored as electron transport material for OLEDs.
However, recent scientific evidence showed that an OLED with Alq3 as the emissive layer and/or host material has intrinsic instability due to the poor hole injection ability of Alq3. In addition, current findings show that Alq3 has little electrochemical reversibility even under normal electrochemical reduction process.
Accordingly, because of consumer expectations of good efficiency, long lifetime and pure color for OLEDs, a need still exists for development of suitable materials used for OLEDs.